The subject matter disclosed herein relates generally to particle accelerators, such as cyclotrons, and more particularly to particle accelerators that direct a beam of charged particles through a stripper foil that removes electrons from the charged particles.
Particle accelerators can be used to generate radionuclides for medical therapy and imaging and also for research in medicine and physics. A cyclotron is a type of particle accelerator and may be one part of a larger nuclide production system. Like other particle accelerators, the cyclotron accelerates a beam of charged particles (e.g., H-ions) and directs the beam into a starting material to generate the isotopes. The cyclotron is a complex system that uses electrical and magnetic fields to accelerate and guide the charged particles along a predetermined orbit within an evacuated acceleration chamber. When the beam of charged particles reaches an outer portion of the orbit, the beam of charged particles passes through a sheet of material (referred to as a “stripper foil”) that removes electrons from the charged particles. The beam of particles, no longer guided by the electrical field, exits the orbit toward, for example, a target assembly.
The target assembly for nuclide production includes a chamber (referred to as a production chamber) that holds a starting material, which may be a liquid, gas, or solid material. The target assembly has a beam passage that receives the beam and permits the beam to be incident on the starting material in the production chamber. To contain the starting material within the production chamber, the beam passage is separated from the production chamber by another sheet of material (referred to herein as a “target foil”). More specifically, the production chamber may be defined by a void within a target body. The target foil covers the void on one side. The particle beam passes through the target foil and is incident upon the starting material within the production chamber.
In many cases, another foil (referred to herein as a “front foil”) may be used. The front foil is sometimes described as a “degrader foil” or “vacuum foil.” The front foil is positioned downstream from the stripper foil, but upstream from the target foil such that the beam of particles intersects the front foil prior to intersecting the target foil. The front foil reduces the energy of the particle beam and separates the target assembly from the vacuum of the cyclotron.
Each of the various foils may consist of only a single material composition (e.g., a single layer of the same material). Target foils may comprise two or more layers (e.g. metal sheet coated with another layer). Due to different purposes and environments, the foils often have different qualities, such as different thicknesses and type(s) of material. For example, target foils can experience an elevated pressure along the side of the target foil that borders the production chamber. Target foils may also experience a corrosive and oxidizing environment due to contact with the starting material. The elevated temperatures and pressures cause stress that renders the target foil vulnerable to rupture, melting, or other damage. Target foils may also contaminate the target media when the ions from the target foil are absorbed by the starting material. The front foils may be configured to, among other things, reduce the energy of the particle beam by a designated amount.
Stripper foils are also susceptible to degradation. Graphite foils, for example, have been used as electron strippers to convert negatively-charged hydrogen ions to protons. Over time, however, cyclical ion-beam exposures cause the graphite foils to wrinkle and/or fracture and become unsuitable for use. A stripper foil with a longer lifetime would reduce downtime of the nuclide production system and lower overall costs for operating the system while also reducing radiation exposure to service personnel.